Coiled Heat Exchanger Having Different Tube Diameters

ABSTRACT

A coiled heat exchanger having a plurality of tubes which are wound around a core tube is disclosed. The heat exchanger having a casing which delimits an outer space around the tubes, where the tubes of a first tube group have a first inner diameter and a first outer diameter and the tubes of a second tube group have a second inner diameter and a second outer diameter. The second inner diameter is different from the first inner diameter and/or the second outer diameter is different from the first outer diameter.

BACKGROUND AND SUMMARY OF THE INVENTION

This application claims the priority of International Application No. PCT/EP2006/006626, filed Jul. 6, 2006, and German Patent Document No. 10 2005 036 414.4, filed Jul. 29, 2005, the disclosures of which are expressly incorporated by reference herein.

The invention relates to a coiled heat exchanger having a plurality of tubes which are wound around a core tube, having a casing which delimits an outer space around the tubes.

Natural gas is continuously liquefied in large quantities in LNG baseload systems. Most of the time, liquefaction of the natural gas is accomplished by heat exchange with a coolant in coiled heat exchangers. However, many other applications of coiled heat exchangers are also known.

In a coiled heat exchanger, several layers of tubes are spirally wound on a core tube. A first medium is piped through the inside of at least one portion of the tubes, and this medium exchanges heat with a second medium flowing in the outer space between the tubes and a surrounding casing. The tubes are merged into several groups on the upper ends of the heat exchanger and fed out of the outer space in a bundled manner.

These types of coiled heat exchangers and their application, for example for liquefaction of natural gas, are described in each of the following publications:

-   -   Hausen/Linde, Cryogenic Engineering, 2^(nd) ed., 1985, pages         471-475;     -   W. Scholz, “Coiled Tube Heat Exchangers,” Linde Reports on         Science and Technology, No. 33 (1973), pages 34-39;     -   W. Bach, “Offshore Natural Gas Liquefaction with Nitrogen Cold     -   Process Design and Comparison of Coiled Tube and Plate Heat         Exchangers,” Linde Reports on Science and Technology, No. 64         (1990), pages 31-37;     -   W. Forg et al., “A New LNG Baseload Process and Manufacturing of         the Main Heat Exchanger,” Linde Reports on Science and         Technology, No. 78 (1999), pages 3-11 (English version: W. Forg         et al., “A New LNG Baseload Process and Manufacturing of the         Main Heat Exchanger,” Linde Reports on Science and Technology,         No. 61 (1999), pages 3-11);     -   DE 1501519 A;     -   DE 1912341 A;     -   DE 19517114 A;     -   DE 19707475 A; and     -   DE 19848280 A.

Tubes with uniform cross sections are used in the case of known coiled heat exchangers.

The invention is based on the objective of further optimizing these types of coiled heat exchangers, in particular in terms of weight, number of tubes, process conditions and/or operating safety.

This objective is attained in that the tubes of at least two tube groups have different outer diameters and/or different inner diameters. A “tube group” in this case is composed of at least one, preferably a plurality of tubes. The tubes in a tube group may, but must not, be adjacent in the tangential and/or radial direction. The two tube groups are preferably located in the same tube bundle. A “tube bundle” describes the entirety of an inner component of a coiled heat exchangers comprised of core tube, the tube layers coiled thereon and intermediate auxiliary means such as connecting pieces etc., which are produced by the coiling process. A coiled heat exchanger has one or more of these types of tube bundles within a casing.

In this way, the tube geometry can be adapted better to specific process-related requirements. These types of specific requirements can consist for example of different thermal properties of various process fractions, which flow through the corresponding tube groups, or even of the different lengths of tubes in different tube layers. Another advantage is that the wall thicknesses can be adapted to different process pressures of the media flowing through the tubes and thereby reduce the weight.

In terms of the invention, the following combinations of the geometric parameters of the tubes of the two tube groups are possible:

Outer Diameter Inner Diameter Wall Thickness Same Different Different Different Same Different Different Different Same or different

“Different” should be understood in this case as a deviation in the corresponding dimension, which is considerably greater than the manufacturing tolerance in effect for it. A parameter is considered “different” from another one if its value deviates by at least 2%, preferably at least 5%.

In terms of the invention, the inner diameter in particular can be varied, preferably with the outer diameter remaining the same.

For example, the pressure drop along the tubes can be influenced by varying the inner diameter. As a result, two different tube groups can be optimized independent of one another for two different process fractions. Basically, this can be accomplished with the same wall thickness, i.e., the two tube groups also have different outer diameters. Alternatively, all tubes can have the same outer diameter; then only the wall thickness and the inner diameter vary.

Therefore, in terms of the invention, it is beneficial in many cases to provide the two tube groups with the same outer diameter and to influence the inner diameter by using different wall thicknesses. As a result, two tube groups with different inner diameters can be coiled on the same tube layer with two different process fractions flowing through them. As compared to allocating the different process fractions to different tube layers, this produces improved uniform distribution of the heat flow in the heat exchanger.

A difference in the wall thickness can be realized when using the same material or even when using different materials (for example aluminum and steel) for the two tube groups. The use of different materials is described in detail in German Patent Application 102005036413.6.

The two tube groups can be arranged in the same or in different tube layers. Of course, more than two tube groups with different dimensions can also be provided. For example, a first and a second tube group can be arranged within a first tube layer and a third tube group can be arranged in a second tube layer.

It is beneficial if two tube groups have different wall thicknesses, particularly to adapt to process fractions having different pressures, for which the two tube groups are intended. A lower wall thickness is used for the tube group with the lower design pressure thereby reducing weight. Depending upon the desired pressure loss and manufacturing-related possibilities, either the inner diameter or the outer diameter of the two tube groups can be different; alternatively, both diameters can be different.

Basically, it is also possible to vary the inner and/or outer diameter of the same tube within the heat exchanger, for example in order to adapt better to the volume of a process stream being vaporized or condensed. In this case, the first tube group includes for example a first section of tubes and the second tube group another one, for example a section of the same tubes adjacent to the first section.

In addition, the invention relates to the application of this type of heat exchanger for executing an indirect heat exchange between a hydrocarbonaceous stream and at least one heat fluid or cold fluid.

The hydrocarbonaceous stream in this case is formed by natural gas for example.

The hydrocarbonaceous stream is liquefied, cooled, heated and/or vaporized during the indirect heat exchange. The heat exchanger is preferably used for natural gas liquefaction or natural gas vaporization.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates an embodiment of a coiled heat exchanger in accordance with the principles of the present invention.

DETAILED DESCRIPTION OF THE DRAWING

The invention and additional details of the invention are explained in greater detail in the following on the basis of an exemplary embodiment depicted schematically in the drawing. In this case, the drawing depicts an inventive coiled heat exchanger 1 for liquefying a stream of natural gas 2 into liquefied natural gas (LNG=liquefied natural gas) 3 by indirect heat exchange with three refrigerant streams, a low-pressure refrigerant 4, a first high-pressure refrigerant 5 and a second high-pressure refrigerant 6.

The coiled heat exchanger in this case features a single tube bundle with three tube groups. The tubes in the tube groups are spirally wound on a common core tube in an alternating manner in different layers. (The tube coiling corresponds to the generally known principle of a coiled heat exchanger; as a result, the geometric arrangement is not depicted in the schematic drawing.) The tube groups in this example are divided by process streams. The natural gas 2 flows through the tubes of a first tube group 7; one of the two high-pressure refrigerants 5, 6 flows through each of the tubes of a second or third tube group 8, 9. The high-pressure refrigerants in this case are guided from the bottom to the top, i.e., in parallel flow with the natural gas. The low-pressure refrigerant 4 flows from the top to the bottom, i.e., in the opposite direction of flow of the natural gas, through the outer space of the tubes and is vaporized in the process. Vaporized low-pressure refrigerant 10 is withdrawn again from the outer space at the lower end of the heat exchanger.

In a concrete numerical example, the process pressures are as follows:

Natural gas 2 120 bar  Low-pressure refrigerant 4 15 bar First high-pressure refrigerant 5 60 bar Second high-pressure refrigerant 6 60 bar

The tubes are manufactured of a light metal material, for example aluminum or an aluminum alloy, and have different wall thicknesses depending on the tube group. In this case, the outer diameters of the tubes in all tube layers are the same.

The wall thicknesses are as follows in a first variant which was optimized in term of weight:

Tube group 7 1.4 mm Tube groups 8 and 9 0.9 mm

In another variant, the wall thicknesses were optimized with respect to the thermal and hydraulic design and with respect to a tube bundle that is structured as homogenously as possible, wherein process-related parameters (e.g., predetermined maximum pressure drops in the individual process streams) were to be complied with. The wall thicknesses are as follows in this second variant:

Tube group 7 1.4 mm Tube groups 8 and 9 1.2 mm

In the second variant, identical tube lengths were achieved in the individual tube groups, whereby the heat exchanger was optimized both in terms of heat transfer as well as in terms of economic efficiency. 

1-8. (canceled)
 9. A coiled heat exchanger having a plurality of tubes which are wound around a core tube, having a casing which delimits an outer space around the tubes, wherein the tubes of a first tube group have a first inner diameter and a first outer diameter with a resulting first wall thickness and the tubes of a second tube group have a second inner diameter and a second outer diameter with a resulting second wall thickness, wherein the second inner diameter is different from the first inner diameter and/or the second outer diameter is different from the first outer diameter and wherein the second wall thickness is different from the first wall thickness.
 10. The heat exchanger according to claim 9, wherein the second inner diameter is different from the first inner diameter and the second outer diameter is a same as the first outer diameter.
 11. The heat exchanger according to claim 9, wherein the first and the second tube groups are arranged within a same tube layer.
 12. The heat exchanger according to claim 9, wherein the first and the second tube groups are arranged in different tube layers.
 13. The heat exchanger according to claim 9, wherein the tubes of the first tube group and the tubes of the second tube group are located in a same tube bundle.
 14. An application of the heat exchanger according to claim 9 for executing an indirect heat exchange between a hydrocarbonaceous stream and at least one heat fluid or cold fluid.
 15. The application according to claim 14, wherein the hydrocarbonaceous stream is formed by natural gas.
 16. The application according to claim 14, wherein the hydrocarbonaceous stream is liquefied, cooled, heated and/or vaporized during the indirect heat exchange.
 17. A coiled heat exchanger, comprising: a core tube; a first tube group spirally wound on the core tube, wherein a tube of the first tube group has a first inner diameter and a first outer diameter with a resulting first wall thickness; a second tube group spirally wound on the core tube, wherein a tube of the second tube group has a second inner diameter and a second outer diameter with a resulting second wall thickness; wherein the second inner diameter is different from the first inner diameter and/or the second outer diameter is different from the first outer diameter and wherein the second wall thickness is different from the first wall thickness; and a casing which defines an outer space around the core tube and the first and second tube groups.
 18. The coiled heat exchanger according to claim 17, wherein the first and second tube groups are each comprised of a plurality of tubes.
 19. The coiled heat exchanger according to claim 17, wherein the first tube group is comprised of a first material, wherein the second tube group is comprised of a second material, and wherein the first material is different from the second material.
 20. The coiled heat exchanger according to claim 17, wherein the first and second tube groups are disposed in a same tube layer around the core tube.
 21. A method of liquefying a stream of natural gas, comprising the steps of: flowing the stream of natural gas through a first tube group of a coiled heat exchanger; and flowing a refrigerant through a second tube group of the coiled heat exchanger; wherein a cross-section of a tube of the first tube group is different from a cross-section of a tube of the second tube group.
 22. The method according to claim 21, wherein the first tube group is comprised of a first material, wherein the second tube group is comprised of a second material, and wherein the first material is different from the second material. 